This invention relates generally to a fluorescent lamp ballast and, more particularly, to a scheme for sensing ballast lamp current.
Conventional liquid crystal display (LCD) backlighting for a laptop computer is provided by a fluorescent lamp partially covered by a shield. The shield serves, in part, to redirect light produced by the lamp toward the LCD. The lamp is powered by a dimmable cold cathode fluorescent lamp (CCFL) ballast which includes a transformer. Since the shield can come into contact with various parts of the laptop including, but not limited to, the ballast inverter, the shield is connected to a reference bus (hereinafter referred to as "ground") for safety purposes.
The CCFL ballast typically includes circuitry for sensing of lamp current to monitor lamp current conditions. The sensing circuitry, which includes a sensing element between the lamp and ground, should sense lamp current under all lamp conditions to ensure stable lamp operation, free of flicker. The sensed lamp current serves as a feedback signal to a controller for driving the ballast inverter.
Parasitic capacitances associated with the shield and transformer make such sensing difficult. More particularly, current can flow from the high voltage terminal of the lamp through the lamp glass to the shield and ground so as to bypass the sensing element. Under such conditions, lower than actual lamp current is sensed making lamp control far more difficult.
Accordingly, it is desirable to provide a CCFL ballast which operates the lamp under more stable, flicker free conditions. The improved lamp sense circuitry should particularly address the parasitic capacitance affect attributable to the shield and transformer.